Venting Crawl Spaces

A crawl space is on the front lines for accumulating ground moisture and being threatened by mold, mildew and rot since it is in direct contact with the ground. Consequently venting crawl spaces is necessary to protect the home from structural damage and the threat of molds and mildew.

Normally a crawl space has several rectangular vents dispersed around the crawl space. The crawl space vents are designed to be opened and closed. Frequently I get asked the question on when should they be opened. The answer is tricky and really depends upon where you live and weather conditions. The theory behind venting crawl spaces is to open them when the outside air is warm and moist, and close them when the air is cold and dry. The thought process is that by opening the crawl space vents during warm and moist weather conditions the moist air will exit the home. However, I have to say that I have been in many crawl spaces during hot summer days, when the crawl space vents are open, and they still feel very damp.

There are powered crawl space vents with motorized fans that can help to draw the moist air out, but in my opinion they are just a more expensive solution for treating a symptom rather than the actual problem.

To really solve crawl space moisture problems, the ultimate solution is to prevent the ground moisture from actually entering the crawl space altogether. The best way to do this is to install a vapor retarder, or moisture barrier, on the floor of the crawl space. The moisture barrier should partially fold up the sides of the basement crawl space and be fastened to the basement walls with either a special tape or with pressure treated framing lumber to create a tight seal. Additionally, sand and then gravel should be installed over the vapor retarder to allow you to move on the crawl space floor without damaging the vapor retarder.

By installing a vapor retarder you can effectively eliminate the bulk of the ground moisture permeating up from the crawl space floor, and as a result eliminate the need for venting the crawl space altogether. This said, I would still suggest including the crawl space vents in conjunction with the moisture barrier. By doing both you will eliminate the threat of crawl space moisture and be sure to satisfy your local building inspector.